Posidonia oceanica meadows are vital components of the marine carbon sink, playing a key role in carbon sequestration and storage and, therefore, in climate change mitigation. The species forms unique structures known as mattes, characterized by the complex intertwining of the sediment matrix, seagrass shoots, and rhizomes, creating a low-oxygen environment. Beyond stabilizing the seafloor, their primary ecological importance lies in the long-term accumulation of organic carbon at high densities. Nonetheless, the carbon storage potential of P. oceanica mattes remains insufficiently assessed. This study focuses on mapping the matte system and estimating the associated organic carbon stock of P. oceanica meadows in the Gulf of Sapri (Southern Italy). In particular, by applying the United Nations’ “System of Environmental Economic Accounting – Ecosystem Accounting” (SEEA-EA) framework, we implemented an integrated assessment that addresses in both ecological and economic terms the important role that  P. oceanica plays as a blue carbon ecosystem. Our results reveal an average C_org stock value of 201 ± 62 Mg ha⁻¹ within the top meter of matte, representing a total estimated economic value of over €2.5 million in the study area. The results of this study can be considered in policy decisions for seagrass conservation and restoration and highlight the contribution that  P. oceanica meadows can provide in supporting broader European climate objectives, such as reducing emissions and reaching climate neutrality by 2050. Finally, the study showcases how the SEEA-EA framework can be applied to marine ecosystems to quantify the benefits they provide to human well-being.